The present invention relates to an eddy current sensor having a measuring coil forming a magnetic field in which an object to be identified is insertable.
Eddy current sensors, which by way of illustration are utilized in non-destructive testing are known, for example, from the book "Magnetische und magnetinduktive Werkstoffprufung" by H. Heppner and H. Stroppe, VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 2. Auflage, 1969 or from the article "Elektronik in der zerstorungsfreien Werkstoffprufung" by H. Luz, published in "Der Elektroniker", 1974, issue 9, pages 9 to 11.
The state of the art eddy current sensors are usually provided with a measuring coil, in the magnetic field of which the object to be identified is arranged, as well as with a reference coil. Furthermore, a signal generator, by way of illustration a sine generator, is provided, the output signal of which is applied to the coils. The output signals applied to the two coils are subtracted from each other by means of a subtracter. The output signal from the subtracter is directly applied to the first demodulator and via a 90.degree. phase shifter to a second demodulator, whereby the output signal from the sine generator, which is shifted via a phase shifter with an adjustable phase angle, is applied to the demodulators as an additional signal.
Due to the twofold demodulation, the (amplified) sensor signal is present as a vector in the "complex impedance plane", whereby the magnitude and the phase of the signal may be drawn upon for evaluation.
As, furthermore, the signal from the sine generator is phase shifted by 0.degree..ltoreq..gamma..ltoreq.360.degree. (.gamma. describing the angle about which the axes of the signal plane formed are turned compared to the axes of the impedance plane), in a parallel branch by means of an adjustable phase angle, the desired signal, e.g. the signal of a welded seam, and the interference signal, which by way of illustration may be produced by lifting effects of one or both coils, can lie in any angle .alpha. to one another, the interference signal usually being substantially greater in magnitude than the desired signal.
Customarily, the complex signal is displayed on a screen and visually assessed. In order to be able to identify the interesting signal (desired signal), the angle .gamma. at the phase shifter is set in such a manner that the interference signal comes to lie exactly in the direction of the y axis. "Channel X" is subsequently drawn upon for evaluation, e.g. by means of an "intuitively" set trigger threshold.
The state of the art sensors utilized, by way of illustration, for material testing have a number of disadvantages.
First of all, it should be understood that a visual assessment on a screen is not suitable if the sensor is to be integrated in automated equipment for testing material.
Moreover, in the known sensors only the projection of the desired signal on the x axis is evaluated. In this manner the signal-to-interference ratio to the interference signal and to noise is further reduced.
Moreover, the angle .gamma. has to be precisely adjusted in known sensors. Minimal changes in the properties of the object to be tested-such as occur, by way of illustration, when changing the batch-may result in incorrect identification.
An object of the present invention is to provide an eddy current sensor, which, by way of illustration, can be utilized for material testing, permitting automated evaluation of the signal at full signal-to-interference ratio.
A solution to the aforegoing object in accordance with the present invention and its further embodiments is set forth in the claims hereto.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.